Process for preparing a ceramic foam

ABSTRACT

A ceramic foam material and process for making same having superior strength and durability properties wherein the impregnated foam impregnated with an alumina hydrate binder is fired at an elevated temperature so as to produce a ceramic body characterized by a solid state, sintered ceramic bond.

BACKGROUND OF THE INVENTION

The present invention is drawn to an improved ceramic foam material andprocess for making same and, more particularly, a ceramic foam materialhaving superior mechanical, thermal and chemical properties than ceramicfoam materials heretofore known.

It is known in the art to employ porous ceramic foam materials to filtermolten metal, especially aluminum, as described for example in U.S. Pat.Nos. 3,893,917, 3,947,363, 3,962,081, 4,024,056, 4,024,212, 4,075,303,4,265,659, 4,342,644 and 4,343,704. The production material for thesefilters comprises primarily a phosphate bonded refractory material,having certain other additions, which has been fired to a temperature ofabout 2000° F. in order to mature the bond. See the process described inU.S. Pat. No. 3,962,081. While this type of refractory is suitable foruse in the aluminum industry and easily withstands most aluminum alloyswhich are typically cast at about 1300° F., it is unsuitable for manyother potential applications due to its low strength and poor chemicaldurability. Naturally, it would be highly desirable to develop amaterial which maintains the favorable properties of the ceramic foammaterials heretofore known, namely, high porosity, low pressure drop,high geometric surface area and tortuous flow path, but which overcomesthe above-noted difficulties of strength and chemical durability. Inaddition, it would be desirable to develope a single material whichcould be relatively simply produced and used in a number ofapplications, especially for use in high temperature applications, suchas with ferrous metals and especially in filtration applications.

Accordingly, it is a principal object of the present invention toprovide an improved ceramic foam material and process for making same.

It is an additional object of the present invention to provide animproved ceramic foam material characterized by superior strength andchemical properties as well as improved mechanical and refractoryproperties.

It is a further object of the present invention to provide an improvedceramic foam material which is relatively simple to produce.

It is a still further object of the present invention to provide aceramic foam material suitable for numerous diverse applications, suchas for ferrous filtration.

Further objects and advantages will appear hereinbelow.

SUMMARY OF THE INVENTION

In accordance with the present invention the foregoing objects andadvantages are readily obtained.

The present invention provides an improved ceramic foam material andprocess for making same which is characterized by superior mechanical,thermal and chemical properties.

The improved ceramic foam filter of the present invention is preparedfrom a ceramic slurry containing a gelled alumina hydrate binder and isfor use in filtering molten metal especially in high temperatureapplications such as with ferrous metals. The filter is characterized byhaving an open cell structure with a plurality of interconnected voidssurrounded by a web of said ceramic. The filter is further characterizedby being substantially binder free, i.e., being substantially free ofglassy phases which detract from the strength of the product. Thestructure is characterized by having a solid state, sintered ceramicbond.

In accordance with the process of the present invention a hydrophobic,reticulated organic polymer foam, preferably polyurethane foam, isimpregnated with an aqueous slurry of a thixotropic ceramic compositionincluding a gelled alumina hydrate binder in an amount from 3-15% byweight based on dry materials, dried and heated to remove the organiccomponent therefrom, and fired at an elevated temperature to produce asolid state, sintered ceramic bond. Preferably boehmite, Al₂ O₃.H₂ O, isthe alumina hydrate. A colloidal dispersion of the hydrate is made withan acid that will volatilize under firing temperature, such as nitricacid, and a thixotropic slurry is made with the desired ceramicmaterial. In the preferred embodiment the firing is at a temperature ofabout greater than or equal to 2000° F. for from 15 minutes to 10 hours,and preferably about greater than or equal to 2500° F., so as tocompletely volatilize any organic materials and then sinter therefractory material. The resultant ceramic foam material issubstantially free of phosphate or other organic materials which aresusceptible to chemical attack. In addition, the resultant material ischaracterized by an increase in strength properties and chemicaldurability. The preferred refractory material is alumina; however, otherrefractory materials such as zirconia, chromia, cordierite, mullite,etc., can readily be used. Naturally, specific temperatures will varyfrom system to system.

DETAILED DESCRIPTION

In accordance with the present invention the ceramic foam is preparedfrom an open cell, preferably hydrophobic flexible foam material havinga plurality of interconnected voids surrounded by a web of said flexiblefoam material. Typical material which may be used include the polymericfoams such as polyurethane foams, and the cellulosic foams. Generally,any combustible organic plastic foam may be used which has resilienceand ability to recover its original shape. The foam must burn out orvolatilize at below the firing temperature of the ceramic material whichis employed.

The aqueous ceramic slurry which is employed should be thixotropic, havea relatively high degree of fluidity and be comprised of an aqueoussuspension of the ceramic intended for use in the material. Typicalceramic materials which may be employed include preferably alumina andalso others such as zirconia, cordierite, mullite, etc.

It is an advantage of the present invention that the use of a phosphateor other detrimental inorganic binder is unnecessary. This results inexcellent high temperature properties because additives tend to reducethe high temperature properties. On the other hand, the process of thepresent invention produces a solid state, sintered product.

In accordance with the present invention alumina hydrate, preferablyboehmite, but also mono- and tri-hydrates, is used as a temporary binderand rheological agent. First a colloidal dispersion is made with an acidthat substantially volatilizes under firing temperature, preferablynitric acid but also hydrochloric, sulfuric or others. As indicatedabove this is a considerable advantage as no residual materials are leftsuch as phosphate that are potentially reactive with molten metals. Thisis followed by the preparation of a thixotropic slurry with the desiredceramic material, preferably alumina.

Generally, one uses from 3-15% alumina hydrate by weight based on drymaterials, and a hydrate:acid ratio of from 2:1 to 5:1, preferably about3:1, all based on 70% acid. In the subsequent preparation of thethixotropic slurry one can use small amounts of organic additives ifdesired, for example, rheological agents, supplemental binders,dispersants and the like. The water content is not especially critical,for example 10-50% water based on total weight can readily be used. Thewater component simply obtains reasonable fluidity to impregnate thefoam and coat the fibers.

Thus, in accordance with the processing of the present invention onefirst gels the hydrate with the acid and adds the ceramic components andwater to form the slurry. If desired, one could admix all componentstogether. The slurry is then used to prepare the ceramic foam filter.

Detailed procedures for preparing ceramic foams for molten metal filtersare described in U.S. Pat. Nos. 3,962,081, 4,075,303 and 4,024,212, thedisclosures of which are hereby incorporated by reference.

The flexible foam material is impregnated with the aqueous ceramicslurry so that the fiber-like webs are coated therewith and the voidsare filled therewith. Normally, it is preferred to simply immerse thefoam in the slurry for a short period of time sufficient to insurecomplete impregnation of the foam.

The impregnated foam is then compressed to expel a portion of the slurrywhile leaving the fiber-like web portion coated therewith and with aplurality of blocked pores throughout the body to increase flow pathtortuosity, i.e., homogeneously distributed throughout the ceramic bodyrather than grouped together. In a continuous operation one may pass theimpregnated foam through a preset roller to effect the desired expulsionof slurry from the foam and leave the desired amount impregnatedtherein. Naturally, this may be done manually by simply squeezing theflexible foam material to the desired extent. At this stage, the foam isstill flexible and may if desired be formed into configurations suitablefor specific filtration tasks, i.e., into curved plates, hollowcylinders, etc. It is necessary to hold the formed foam in position byconventional means until the organic substrate is decomposed, orpreferably until the ceramic is sintered. The impregnated foam is thendried by any suitable means, such as air drying, accelerated drying at atemperature of from 100° to 700° C. for from 15 minutes to 6 hours, orby microwave drying. Air drying may be achieved in from 8 to 24 hours.After drying, the material is heated at an elevated temperature tosinter the ceramic coating on the fiber-like webs leaving the pluralityof blocked pores as described above.

In accordance with the present invention, the drying procedure firstyields crystallites or alumina, initially gamma-alumina eventuallytransformed to alpha-alumina. This provides sufficient green strengthfor handling and firing.

The actual firing conditions depend on the ceramic. Generally,temperatures in excess of 2000° F. and preferably in excess of 2500° F.for at least 15 minutes and generally at least 1 hour at temperature andgenerally less than 10 hours in order to volatilize the web of flexiblefoam and sinter the ceramic to form the solid state sintered ceramicbond.

The resultant product is a porous, fused ceramic foam material which issubstantially free of organic components, and is characterized bysuperior mechanical, thermal and chemical properties to ceramic foammaterials heretofore known. The ceramic foam is characterized by havingan open cell structure with a plurality of interconnected voidssurrounded by a web of ceramic with the substantial absence ofpotentially harmful binders or glassy or clay phases. The foam is asolid state, sintered product especially useful for high temperatureapplications such as ferrous or steel filtration. The aforesaidstructure results in increasing desirable physical properties, such asincreased mechanical, thermal and chemical properties. Any small amountsof organic additives would be driven off in the firing process. Thesmall amounts of inorganic additives that may be used if desired wouldnot detract from properties, for example, less than 1% by weightsintering aids, such as zinc oxide, grain growth inhibitors, such asmagnesium oxide, or inorganic rheological aids, such as clays.

In accordance with the present invention, the specific features thereofwill be more readily understandable from a consideration of thefollowing data.

EXAMPLE I

A thixotropic ceramic slurry was prepared by first preparing a gelconsisting of the following:

1853 grams of boehmite

476 milliliters of concentrated nitric acid

9060 milliliters of water.

This gel was added to a dry powder blend of:

74 kilograms of alumina

79 grams of magnesia.

An additional 300 milliliters of water was added to this mixture and theentire batch was thoroughly mixed in a high speed intensive mixer.

The aforesaid thixotropic ceramic slurry was used to impregnate nominal20 pore per inch, open cell, flexible polyurethane foam blocks to agreen density of about 10% of theoretical so that the fiber-like webs ofthe foam were coated therewith and the voids filled therewith.Impregnation was accomplished by immersing the foam in the slurry andusing pre-set rolls to compress the foam and expel a portion of theslurry while leaving the fiber-like web portion coated therewith andwith a plurality of blocked pores throughout the body to increase flowpath tortuosity.

The resultant impregnated foams were dried and heated to remove theorganic component therefrom and fired to produce a solid state, sinteredceramic bond by heating to approximately 3000° F. for one (1) hour. Theresultant product is a porous, fused ceramic foam material substantiallyfree of organic components and characterized by excellent mechanical,thermal and chemical properties. The linear firing shrinkage was about15% such that the resulting fired ceramic foam had a fired nominal poresize of 23 pores per inch and a fired density of about 15% oftheoretical.

EXAMPLE II

Ceramic foams prepared in accordance with Example I and having fireddimensions of 2"×2"×1" were used in an investment casting operation tofilter high quality stainless steel castings. The ceramic filters wereplaced directly in the mold cavity and were under poured. The resultingcastings were substantially free of inclusion related defects.

EXAMPLE III

Ceramic foams prepared in accordance with Example I and having fireddimensions of 4"×4"×1" were used to filter stainless steel fan housings.In this case a top pour arrangement was employed. High quality castingswere obtained. Weld repair, normally required for such castings andwhich is generally due to inclusions, was substantially eliminated.

EXAMPLE IV

Ceramic foams prepared in accordance with Example I and having fireddimensions of 4"×4"×1" were used to filter a high temperaturenickel-aluminum-bronze alloy test casting. A high quality casting wasobtained. This casting would normally require many hours of weld repair;however, the resultant casting of this example reduced the weld worksubstantially.

EXAMPLE V

A thixotropic slurry was prepared as in Example I and was used toimpregnate a nominal 10 pore per inch, open cell, flexible polyurethanefoam block as in Example I. the resultant impregnated foam was microwavedried and then fired to 3000° F. to produce a solid state, sinteredceramic bond and a porous, fused ceramic foam material substantiallyfree of organic components characterized by excellent mechanical,thermal and chemical properties. The fired ceramic foam, measuring4"×4"×1" with a fired pore size of about 12 pores per inch, was used tofilter a low alloy steel casting. Inclusion related defects weresubstantially eliminated.

Thus, as can be seen from the foregoing, a ceramic foam material havingsuperior strength properties and chemical durability is obtained.

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiment is therefore to be considered as in allrespects illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims, and all changes which comewithin the meaning and range of equivalency are intended to be embracedtherein.

What is claimed is:
 1. A process for preparing a ceramic foam filter forfiltering molten metal which comprises: providing a reticulated, organicpolymer foam; impregnating said foam with an aqueous slurry of athixotropic ceramic composition including a gelled alumina hydratebinder in an amount of from 3 to 15%; drying and heating saidimpregnated polymer foam to remove the organic component therefrom; andfiring at an elevated temperature to produce a solid state, sinteredceramic bond characterized by being substantially binder free andsubstantially free of glassy phases.
 2. A process according to claim 1wherein the polymer foam is a polyurethane foam.
 3. A process accordingto claim 1 wherein boehmite is the alumina hydrate.
 4. A processaccording to claim 1 wherein a colloidal dispersion of the hydrate ismade with an acid that will volatilize under firing temperature and athixotropic slurry is made with the ceramic material.
 5. A processaccording to claim 4 wherein said acid is nitric acid.
 6. A processaccording to claim 4 wherein the hydrate to acid ratio is from 2:1 to5:1 based on 70% acid.
 7. A process according to claim 6 wherein thehydrate to acid ratio is about 3:1.
 8. A process according to claim 6wherein the hydrate is first gelled with the acid followed by admixingthe gelled material with the ceramic and water to form the slurry.
 9. Aprocess according to claim 8 wherein all components are admixedtogether.
 10. A process according to claim 1 wherein the firing is at atemperature of at least 2000° F. for at least 15 minutes.
 11. A processaccording to claim 10 wherein the firing is from 15 minutes to 10 hours.12. A process according to claim 10 wherein the firing is at atemperature of at least 2500° F.
 13. A process according to claim 10wherein the firing temperature is approximately 3000° F.